1. Field of the Invention
The present invention relates to a process for preparing polyester such as polyethylene naphthalate (hereinafter as PEN) polymers. More particularly, the present invention relates to a novel and improved process for preparing PEN based polymers by adding a primary alcohol into the esterification process of naphthalene dicarboxylic acid (hereinafter as NDCA) and ethylene glycol (hereinafter as EG).
2. Description of the Background Art
PEN which is currently industrially manufactured has a number of superior properties to polyethylene terephthalate (hereinafter as PET). PEN has a higher degree of crystallization, a higher softening point, and other various superior properties in terms of mechanical strength, resistance to gas permeability, chemical resistance, thermal resistance, weather resistance, electric insulation, etc. Consequently, such PEN is widely used for manufacturing high quality films, bottles, high strength fibers and other industrial materials. PEN can be prepared by using naphthalene dicarboxylic acid (NDCA) or naphthalene dicarboxylic acid dimethylester (will be referred to as NDC hereafter) with ethylene glycol.
Commercially, PEN is prepared by a two step reaction consisting of 1) esterification between NDCA and glycol to obtain a low molecular weight esterified compound; and 2) polycondensation reaction of the esterified compound to obtain a higher molecular weight PEN product. Although the reaction scheme and apparatus required for producing PEN is similar to those for PET, detailed reaction conditions may vary according to the raw materials employed.
When NDC and EG are used as the raw materials to prepare PEN, the first step is to produce bis (beta-hydroxyethyl) naphthalate or its low molecular weight prepolymer (hereinafter as esterified compound) by the esterification of NDC and EG in the presence of zinc acetate [Zn(OAc).sub.2 ] or manganese acetate [Mn (OAc).sub.2 ] at a reaction temperature ranging from 180 to 260.degree. C. to remove methanol. In this case, NDC and EG are admixed to form a slurry before being injected to an esterification reactor. The second step is to prepare a high molecular weight PEN polymer by the polycondensation reaction of the esterified compounds in the presence of polymerization catalysts such as antimony trioxide (Sb.sub.2 O.sub.3) at a reaction temperature ranging from 280 to 300.degree. C. at a reduced pressure (generally less than 1.0 torr).
Meanwhile, when NDCA and EG are used as the raw materials for manufacturing PEN, a slurry of NDCA and EG might be esterified to produce bis (beta-hydroxyethyl) naphthalate or its low molecular weight esterified polymers without using a catalyst such as zinc acetate [Zn (OAc).sub.2 ] or manganese acetate [Mn (OAc).sub.2 ] because NDCA itself acts as an acid catalyst promoting esterification reaction similar to that of TPA in producing PET. Appropriate reaction temperature range is from 180 to 260.degree. C. In this case, water is formed as an esterification side product instead of methanol when NDC and EG are used as the raw materials. NDCA and EG should be admixed to form a slurry before being injected to an esterification reactor. However, forming a slurry requires an excess amount of EG to attain fluidity. After the esterification compounds are produced from NDCA and EG, they can be readily polycondensed in the presence of a polymerization catalyst such as antimony trioxide at a reaction temperature ranging from 280 to 300.degree. C. at a reduced pressure (generally less than 1.0 torr) to produce a high molecular weight polymer in the same manner as when NDC and EG are used as the raw materials.
Generally, PEN has naphthalene rings in the molecular structure and a higher melt viscosity than PET, and requires a higher polymerization temperature. Therefore, PEN is more subject to discoloring by impurities and oxidation than PET. Besides, NDCA or NDC has a higher molecular weight, a lower solubility in EG than TPA or DMT used in the PET production process, the process conditions of PEN are different from those of PET. For instance, a slurry of the EG and TPA mixture having a molar ratio of EG/TPA ca. 1.1.about.1.2 can be easily fed to the esterification reactor continually. Meanwhile, to produce an EG/NDCA slurry equivalent to that of an EG/TPA slurry for PET process, a molar ratio of EG/NDCA must be greater than 3.0, since it is practically impossible to feed an EG/NDCA slurry having a lower molar ratio than 3.0 into the esterification reactor.
However, it is well known that an excess amount of EG in an EG/NDCA slurry decreases productivity and requires a higher separation cost to remove EG from the product. In addition, because an excess amount of EG lowers the reaction rates and lengthens the reaction time, formation of a side product, diethyleneglycol (hereinafter as DEG), which is detrimental to the quality of the final product is increased. Consequently, it is very important to minimize the use of EG in order to obtain a good quality PEN product.
Because the slurry formation tendency of the NDCA with EG depends not only on the molar ratio of NDCA/EG but also on the NDCA particle sizes, excess amount of EG required for a slurry may be reduced as NDCA particle size increases. However, since the reaction occurs on the NDCA particle surface or in the melt state, NDCA particles of smaller sizes are desirable for forming a uniform slurry and higher reaction rates. Therefore, it is difficult to minimize an excess amount of EG simply by controlling the NDCA particle sizes. Rather, it is required to develop alternative methods to make a slurry of NDCA and EG using the minimal amount of EG.
In order to overcome these problems, many methods have been proposed to add water into EG (U.S. Pat. No. 5,811,513 and WO 90-14375, WO 97-17391). However, these methods mentioned in the above references still employ too much excessive amount of EG and water with respect to NDCA or use a full-batch process instead of a continuous process. Therefore, these methods are not adequate for producing high quality PEN products from NDCA and EG.